docs: document `log` trait on `Application` class

The docs for the [Application](https://traitlets.readthedocs.io/en/latest/config-api.html#traitlets.config.Application) class don't mention the default value of the `log` trait on `Application` to be a `Logger` instance named with `self.__class__.__name__`. I had to dig through the code to find [the line where this default is set](https://github.com/ipython/traitlets/blob/main/traitlets/config/application.py#L286). Can we update the documentation here accordingly?

Start with the Application class documentation at the linked config-api page and inspect traitlets/config/application.py around line 286, where the default is set. Update the documentation to state that the log trait defaults to a Logger named with self.__class__.__name__, then check the rendered documentation for the new detail.

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